Excerpt from the writings of John Gill:
(3.) Christ was set up from everlasting, as the Mediator of this covenant: his goings forth, and acting therein, on the behalf of his people, were of old, from everlasting. He then engaged to be a surety for them, and was accepted of by God the Father as such; who thence forward, to use the Doctor's words, just now cited, looked for his debt, and expected satisfaction of him, and let the sinners go free, for whom he engaged. Looking at him for the payment, he looked at them as discharged; and they were so in his eternal mind, and, in this respect, were justified from eternity. And indeed, it is a rule that will hold good, "That as soon as any one becomes a surety for another, the other is immediately freed, if the surety be accepted;" which is the case here. And it is certainly most prudential, when a man has a bad debt, and has good security for it, to have his eye upon the bondsman or surety for payment, and not upon the principal debtor, who will never he able to pay him.
(4.) That as soon as Christ became a surety, the sins of all those persons, for whom he became a surety, were reckoned and accounted to him; and, if accounted to him, then not to them; if they were laid to his charge, then not to theirs; and, if he was answerable for them, then they were discharged from them. If there was an imputation of them to him, then there must he a non-imputation of them to them; which the apostle plainly intimates, when he says, God was in Christ, that is, from everlasting, reconciling the world unto himself, not imputing their trespasses unto them. (2 Cor. 5:9) Witsius, citing this text of scripture, says: "God hath reconciled the whole world of his elect to himself, and hath declared that he will not impute their trespasses to them, and that because of the consummate satisfaction of Christ, 2 Corinthians 5:19, wherefore, says he, I am of opinion, that this act of God may be called the general justification of the elect." Nor ought it to he thought strange, foreign, or far-fetched, that the justification of God's people is inferred from the imputation of their sins to Christ, and the non-imputation of them to them; since the apostle Paul, in Romans 4:6-8, has so manifestly deduced, and strongly concluded the imputation of righteousness, which is the ratio formulis of justification
, from the non-imputation of sin, and remission of it.